There are two trends in distributed system that seem to have no current end to. The first is that malicious attacks are becoming increasingly common. The second is that more and more critical infrastructure, information and business is being digitized and moved on-line. It is generally believed that Byzantine fault tolerant (BFT) replication systems will play a major role in providing a core trust infrastructure that will help these two trends better co-exist.
One of the earliest practical systems proposed is aptly referred to as the practical Byzantine fault tolerant (PBFT) protocol that used a novel algorithm, multiple optimizations, and state-of-the-art cryptographic techniques. PBFT can perform well when tuned to tolerate one failure and can scale to withstand a few failures while maintaining good performance. Zyzzyva is another protocol that systematically exploits speculation to improve both latency and throughput. The replication algorithm in Zyzzyva is optimized for a common mode environment, which is characterized by fault free nodes (but may be slow or have crashed altogether) and synchronous communication. Zyzzyva introduces optimizations aimed to reduce the client latency and allow the client to take a larger role in driving the protocol.
Bitcoin and more Ethereum are distributed crypto-currency computing platforms that have had significant impact on the design of distributed ledger systems. They demonstrate that scaling replication to hundreds or even thousands of nodes is feasible and of potentially significant value to society. A core reason why distributed systems like Bitcoin and Ethereum scale so well is because their replication protocol has a linear message complexity per decision block. To make progress, the node who solves the Proof-of-Work (PoW) puzzle just needs to disseminate its message (that includes a decision block and PoW) to all replicas. Reliance on Proof-of-Work has inherent disadvantages, however: It takes tens of seconds if not minutes to add a new decision block and it takes even longer to commit with high certainty. Moreover, the economics of PoW seem to be causing these systems to be controlled by just a handful of powerful coalitions.
There is a growing interest in replacing the PoW mechanism of today's crypto-currencies with techniques that leverage the throughput advantages of Byzantine fault tolerant replication.